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Extinctions Heal Slowly

The history of life on Earth has been a heated tug of war between forces that power the birth of new species and those that grind life forms into oblivion. Now a new study suggests that while extinction rates vary widely, origination rates march to their own slow drummer. The study, published in the 3 January issue of Nature, fuels fears that it will take millions of years for biological diversity to recover from the current wave of human-caused extinctions.

In recent years scientists have challenged a long-held assumption that evolution's pace can accelerate to fill the biological holes caused by episodic mass extinctions. Last year, a pair of researchers reported that it takes about 10 million years for evolution to renew the planet's biodiversity following an extinction event, whether that event is large or small. Building on that finding, one team member, earth scientist James Kirchner at the University of California, Berkeley, decided to compare rates of extinction to rates of origination over a 500-million-year period.

To compare evolutionary clocks, Kirchner turned to the same database he'd analyzed earlier: the late Jack Sepkoski's encyclopedic compilation of marine animal fossils, which spans a 540-million-year history and lists the first and last known occurrences of about 4500 families and 36,000 genera. To untangle such a massive and uneven database, Kirchner used a specialized form of a mathematical technique called spectral analysis, which was developed by astrophysicists to make sense of the complex light signals received from distant stars. Kirchner concluded that extinction rates are up to two and a half times more variable than origination rates, with originations of new species pulling ahead of extinctions only when researchers look at chunks of time greater than 25 million years. The finding is further proof that "while extinction rates can spike, diversification rates can't rev up to offset them," Kirchner says.

The work supports the emerging view that "origination and extinction are decidedly different processes determined by different rules," says Arnold I. Miller, a paleobiologist at the University of Cincinnati in Ohio. But Douglas Erwin, a paleobiologist at the Smithsonian's National Museum of Natural History in Washington, D.C., says he's "leery about the analytical structure of the study," because it's based on an outdated version of the time scale that accompanies Sepkoski's database. "I'm not sure that the analysis is sufficiently robust to support the conclusions," says Erwin.